The pursuit for fuel-efficient and durable engines has intensified the need to understand frictional losses in valvetrain systems operating mainly under boundary and mixed lubrication. This study introduces an advanced in-situ technique based on giant magneto-resistive (GMR) chip to directly measure independent tappet and shim rotation linked with friction measurement in a production engine head without altering its geometry. Complementary experiments were conducted on a reciprocating rig using real engine components to validate the findings. Tests with SAE 30 base oil and four friction modifiers namely MoDTC, Molyvan, Polymeric, and an Organic friction modifier (OFM) showed that MoDTC consistently delivered the lowest frictional torque but also suppressed tappet rotation, while the polymeric friction modifier yielded the highest tappet rotation with relatively modest friction reduction across all camshaft speeds and lubricant temperatures. The base oil produced the highest friction torque overall but minimal rotation, demonstrating the strong influence of tappet–bore resistance. Shim rotation was generally low but became significant at low camshaft speed and high lubricant temperature. The analysis revealed that nearly 10% of cam–tappet friction contributes to tappet rotation, demonstrating that the equilibrium between cam–tappet and tappet–bore friction governs rotational dynamics. Reciprocating tribometer tests verified the friction reducing performance of MoDTC but showed a different ranking for the other modifiers due to the absence of rotational motion. This suggests that reciprocating sliding tribometers may not fully replicate real engine conditions. The results reveal that friction modifiers influence both tribological losses and the coupled rotational dynamics within the valvetrain.